A method for the electronic control of brake force distribution (EBD control) has been disclosed in the art wherein the brake slip which occurs on the front wheels is compared to the slip of the rear wheels (DE 33 01 948 A1, DE 33 23 402 C2). The rear-wheel brake slip is limited to a defined percentage, for example to 85 to 87%, of the front-axle brake slip by modulation and limitation of the braking pressure introduced into the wheel brakes of the rear wheels. Of course, a method of this type necessitates measuring the rotational behavior of both the front wheels and the rear wheels.
In addition, relatively small rotational speed differences and low slip values must be determined and evaluated for the control of brake force distribution because EBD control takes place already in the partial braking range, that is, at low slip values. Consequently, high demands are placed on the accuracy of determining and evaluating the rotational behavior and the slip values.
DE 44 17 935 A1 (P 7662) discloses a circuit arrangement for an EBD control which comprises circuits by which the deceleration of the rear wheels and the deceleration of the vehicle is determined. The difference between the rear-wheel deceleration and the vehicle deceleration is evaluated for controlling the brake force distribution. This means that deceleration values rather than the slip values are made the basis of the control in this prior art circuit in order to reduce the demands placed on the measuring accuracy of the sensors. However, the system or the circuit, respectively, now as before is based on the determination of the rotational behavior of all vehicle wheels, even though the objective remains that the vehicle deceleration be determined by approximation in the event of failure of a front-wheel sensor.
An object of the present invention is to develop a method of controlling the brake force distribution which manages with one or two rotational speed sensors without having to tolerate a reduced effectivity of the control, i.e., with respect to maintaining driving stability or a shortest possible stopping distance. This is because an excessively high pressure on the rear wheels would jeopardize driving stability and a too low braking pressure would render the stopping distance longer.
This object can be achieved by the present invention wherein after the commencement and/or detection of a braking operation or brake actuation process, a switch-over to maintaining the brake force or the braking pressure on the rear wheels constant is performed, that from the initial vehicle deceleration upon commencement of the braking operation, a vehicle reference quantity or vehicle reference speed is derived by measuring the rotational behavior of the rear wheels, by filtering the measured values and by extrapolation, that the current rotational behavior of the rear wheels is compared with this reference quantity, and that the rotational behavior in comparison with the reference quantity is evaluated for controlling the brake force distribution (EBD control) and/or for anti-lock control of the rear wheels.
Thus, the present invention is based on the knowledge that the need for wheel sensors on the front wheels is eliminated in an EBD control when the rotational speed information acquired on the differential or on the two rear wheels, after extrapolation of the information acquired in the period of measurement, is evaluated to produce a reference quantity, and when the current information determined subsequent to the period of measurement is assessed in the way described and evaluated for the EBD control. Depending on the respective situation, as will be described hereinbelow in detail, the braking pressure on the rear-wheel brakes is then continued to be maintained constant, or further pressure increase on the rear-wheel brakes is allowed. This is the basic principle.
Thus, the commencement of a braking operation or brake actuation process can be identified in a very simple fashion by evaluation of a brake light switch signal or any other signal which is triggered when the brake is actuated. Frequently, it is assumed appropriate to first wait for a short delay period or waiting period between the brake actuation and the determination and detection of the initial vehicle deceleration. This delay period may be in the order of 5 to 30 msec.
The reference quantity or vehicle reference speed is suitably determined after a predetermined measurement period which favorably ranges between 50 msec and 500 msec and starts with the detection of the braking operation or brake actuation process, or upon expiry of the additional delay or waiting period.
According to one embodiment of the present invention, the braking pressure in the rear-wheel brakes is maintained constant, at least in appoximation, until the braking operation is terminated when, upon detection of the brake actuation process and determination of the reference quantity, the wheel deceleration or the difference between the current speed of the respective rear wheel and the reference quantity remain constant or increase to an only relatively small extent. Further braking pressure increase on the rear wheels is allowed when the current wheel deceleration decreases or the current wheel speed approaches the reference quantity.
In another especially favorable embodiment of the method of the present invention, the pressure variation in the rear-wheel brakes is determined in approximation by sensing and evaluating pressure control or pressure adjusting signals and/or the rotational behavior of the rear wheels and by producing a pressure model. To produce the pressure model, especially the actuation times of the inlet and outlet valves are added and evaluated in consideration of the pressure increase and pressure decrease characteristic curves of the system. This permits a quick and relatively precise adjustment of the correct braking pressure in the rear-wheel brakes without additional expenditure in pressure sensors, additional monitoring circuits, etc.